1. Field of the Invention
The present invention relates to distributed and push-pull power transistors, and more specifically to the pre-matching of power transistors.
2. Prior Art
Distributed amplifiers for high-power, high-frequency applications utilize multi-transistor arrays that are represented by distributed large-signal models, such as the one shown in FIG. 1. Each array comprises a plurality of transistors, for example transistors Q1 through QN, connected to allow for high-power operation, each multi-transistor array effectively forming a transistor 110 or 120 having an effective emitter (E) and an effective collector (C).
A simulation of such a circuit reveals significant degradation of the microwave characteristics of the transistor arrays when compared to an ideal power transistor. This degradation is caused by the losses of the transmission lines, for example transmission lines 112-1 through 112-N, connecting the bases of transistor Q1 through QN, and transmission lines 130-1 through 130-M, connecting the transistor arrays, and is best explained by observing the optimum impedances at various locations in the circuit. These locations are shown as Z1 through Z7 in FIG. 1. For the ideal device, increasing the number of transistors results in a decrease of the optimum input matching impedance, which, in this case, also remains capacitive. However, in the case of an actual device, such as the one represented by the model of FIG. 1, the lossy line segments also contribute to this impedance. Therefore the input impedance of transistor Q1 of effective transistor 110, for example, is large compared to the resistance of the line connecting it to transistor Q1 of effective transistor 120. Therefore, the line segment at the base terminal of transistor Q1 of effective transistor 110 will not influence its microwave gain or optimum impedance significantly. Notably, as more transistors are added to the transistor array, for example array 110, the total device impedance decreases. Eventually the total device impedance reaches values that are comparable to the impedance of the transmission lines. At this point, a significant effect is observed on the microwave gain of the transistor array.
This situation is illustrated in FIG. 2 representing the evolution of the transistor optimum input impedances under conjugate matching conditions, Zin,opt (=Z1 through Z7), as well as the maximum available gain (MAG) in various locations. Similarly there is a maximum stable gain (MSG) graph, not shown. It is shown that there is a continuous increase of influence of the lossy lines on the optimum input impedance and MAG as device complexity is increased. Specifically, MAG loss between positions 5 to 6 of the graph is 1.34 dB while it is 2.05 dB between positions 3 and 4. Moreover, while the addition of one array of transistors does not significantly affect MAG when the device array complexity is small (transitions 6-7 and 4-5), this is not the case for transition 2-3 where MAG is reduced by 0.8 dB. Also, the real part of Zin,opt is increasing as the size of the array increases, which is the opposite trend compared to the case of the ideal model. Finally, the imaginary part of Zin,opt gradually becomes inductive as more stages are added due to the increased influence of the parasitic inductances of the transmission lines. This is also contrary to the trend in the case of the ideal situation where no transmission line losses are considered. The above degradation of MAG of large transistor arrays prohibits their exploitation in power applications, especially at high frequencies, for example at frequencies higher than 5.0 GHz.
In view of the deficiencies of solutions suggested by prior art, it would be advantageous to provide a solution that eliminates or at least significantly reduces these negative effects. It would be further advantageous if such a solution would be suitable for high-power, high-frequency circuits, and in particular, those circuits operating at frequencies higher than 5.0 GHz.